Batter shape / cell configuration

[pimpmybike]

Im researching building my own battery pack for the first time and deciding what batteries to try to find and what configuration to use. I have been trying to figure out what max amp for range battery can fit in my old scooter. I have a shwinn e350 scooter. Right now i am running a pack i bought years ago in the back basket which does not fit in the bottom battery compartment with limited shape and size the maximum shape the battery that compartment can hold is rectangle of 305mm (12 inches)x 118mm (4,75 inches) x 68mm (2,75 inches), so i want to build a pack that is equal too or smaller than that size, including the bms. i do not see any battery packs for sale with both the 2.75 inch and 4.75 inch limitations on two of the dimensions. I am building a welder and buy battery cells to make a pack. I do not understand all the lingo of 10S6P configuration or options for the best type of batteries to buy for good price. So researching i have been speaking to ebay sellers. One Chinese seller on ebay says he can configure a pack that will fit for $229.00. he says "We can make the battery smaller than that size with a 20A BMS using new G25 2500mAH 8A 3C The battery model is 36V 15Ah (10S6P) build within 60 pieces of G25 cells. The cells are quality" should i coppy his specs?. Are those good cells? they are hard to find in search engines. not many cells say g25 on them? Do people suggest i copy this kind of 10S6P configuration with this kind of battery? and i guess that is referring to how the # of cells in series and parallel end up near 36v?. i want to choose good cell and know where to purchase the best cells. Im in no rush but really want to hear people opinion. More about my current configuration. Right now I am running a a 36volt 750 watt 16amp pack with 36volt 500 watt brushed controller and and over volted 25v 500 watt motor running at the 36volts.